1. Field of the Invention
The present invention relates to method of lengthening the useful life of heating elements at low temperatures. More specifically, the useful life of heating elements that include molybdenum silicide and molybdenum tungsten silicide, as well as different alloys of these basic materials, is lengthened. Such heating elements are produced by Applicant Sandvik AB in a relatively large number of applications.
2. Description of the Related Art
When such heating elements are operated at relatively low temperatures, for example at temperatures around 400-500xc2x0 C., no protective silica scale (so-called glass layer) will form on the elements, as opposed to when operating the elements at high temperatures. Instead, the elements are subjected to so-called pest, meaning that a non-protective layer of MoO3 and SiO2 forms on the surfaces of the elements. This mixture is porous and readily disintegrates, resulting in a significant shortening of the useful life of the elements.
However, there are applications in which such elements are, nevertheless, the best alternative. One example in this regard is found in the heating of LPCVD-chambers, (Low pressure Chemical Vapor Deposition) in the manufacture of electronic circuits.
The low temperature properties of such heating elements can be improved, by pre-oxidizing the elements at a temperature of about 1500xc2x0 C. or higher, so as to form a skin of SiO2. Such a skin will slow down the formation of pest.
The proposed method greatly lengthens the useful life of such heating elements.
The present invention thus relates to a method of lengthening the useful life of heating elements that are essentially formed from molybdenum silicide and molybdenum tungsten silicide and different alloys of these basic materials, when said elements are operated at a low temperature, such as a temperature in the range of 400-800xc2x0 C. The method includes providing a gaseous atmosphere that surrounds the elements when said elements are operated, wherein the gaseous atmosphere has a water content that is less than about one percent by volume.
The present invention is based on the surprising insight that the oxide products MoO3 and SiO2 are formed to a much less extent when the water content of the gas surrounding the elements is kept to a low level, despite the oxygen content of the surrounding gaseous atmosphere being very high.